1. Field of the Invention
The present invention relates to a power source apparatus to be applied to vehicles.
2. Description of the Related Art
A vehicle with an internal combustion engine generally has a lead-acid battery in order to supply electric power to various types of electrical loads such as a starter motor mounted on the vehicle. The lead-acid battery is cheap in cost when compared with high density energy batteries (high performance batteries) such as nickel batteries and lithium batteries, but has a low durability resistance to frequent charge and discharge. For example, because a lead-acid battery mounted on a vehicle with idle reduction function (which is a function to automatically stop idling to save and reduce fuel consumption) discharges electric power frequently, this causes a rapid deterioration of the lead-acid battery. In particular, a lead-acid battery mounted on a vehicle with an alternator capable of regenerating electric power when the vehicle decelerates is charged with such regenerative electric power frequently, this frequent charge would cause a rapid deterioration. Using a high performance battery to avoid the above drawback of the lead-acid battery would cause a large manufacturing cost.
Conventional techniques disclosed by the following technical documents D1 to D5 have proposed an improved structure where high performance batteries (as secondary battery) with high price and lead-acid battery with low price are mounted in parallel to a vehicle.    D1: Japanese patent laid open publication No. JP 2007-46508;    D2: Japanese patent laid open publication No. JP 2007-131134;    D3: Japanese patent laid open publication No. JP 2008-29058;    D4: Japanese patent laid open publication No. JP 2008-155814; and    D5: Japanese patent laid open publication No. JP 2009-126395.
That is, during idle reduction mode (which is capable to stop the engine during idling in order to reduce fuel consumption), electric power such as regenerative electric power is preferentially supplied to the high performance battery, and electric power of the high performance battery is preferentially supplied to the electrical loads. On the other hand, from the above viewpoint to reduce the electric power consumption while a vehicle stops in a car park for a long period of time, it is controlled so that the lead-acid battery supplies electric power to electrical loads. As described above, a combination of two types of batteries makes it possible to downsize the high performance battery, and to suppress the increase of the manufacturing cost.
By the way, overcharge or over discharge of a battery would cause a rapid deterioration. It is therefore preferable to use the battery within an optimum SOC (state of charge, hereinafter, will be referred to as the “the use range of SOC” or the “usable range of SOC”) which is not overcharged or discharged, where the SOC indicates the charge state of the battery, because an open circuit voltage of the battery corresponds to SOC. In other words, the change of SOC of the battery has a different open circuit voltage of the battery. In general, the open circuit voltage (for example, 12.7 V to 12.8 V) of a lead-acid battery is not equal, within the use range of SOC, to an open circuit voltage of a high performance battery.
Because the lead-acid battery and the high performance battery are connected in parallel in a power source apparatus, a current flows from the battery with a high terminal voltage Vd to the battery with a low terminal voltage when discharging, and this would cause the batteries to be in an over discharged condition which is out from the use range of SOC. In general, the terminal voltage Vd of a battery can be expressed by the following formula (1):Vd=V0−Id×R  (1),where Id is a discharging current of the battery, R is an internal resistance of the battery, and V0 is an open circuit voltage of the battery.
The conventional techniques disclosed by the technical documents D1 to D5 previously described have proposed a structure to use a DC/DC converter which is placed between those batteries such as a high performance battery and a lead-acid battery. This structure can adjust the terminal voltage of the high performance battery, which is higher in terminal voltage than that of the lead-acid battery, by the DC/DC converter, and prevent a current which flows from the high performance battery having a high terminal voltage to the lead-acid battery of a low terminal voltage in order to prevent the lead-acid battery from overcharge.
However, because such a DC/DC converter is a high price device, it is difficult to decrease the total manufacturing cost of a power source apparatus for vehicle which requires a DC/DC converter in order to prevent a lead-acid battery from overcharge.